From Production to Preservation: Hand-Painted Magic Lantern Slides from the National Museum of Natural History and Science

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Beatriz Maria Barata Rodrigues

[Nome completo do autor]

Licenciada em Conservação-Restauro

[Habilitações Académicas]

Setembro 2019

From Production to Preservation: Hand-Painted Magic

Lantern Slides from the National Museum of Natural

History and Science

Dissertação para obtenção do Grau de Mestre em

Conservação e Restauro, especialização em Conservação e Restauro

Dissertação para obtenção do Grau de Mestre em

[Engenharia Informática]

Orientador:

Professora Doutora Márcia Gomes Vilarigues

Professora Auxiliar, Faculdade de Ciências e Tecnologia da

Universidade NOVA de Lisboa

Co-orientador:

Mestre Ângela Barros Santos

Conservadora-Restauradora, Faculdade de Ciências e

Tecnologia da Universidade NOVA de Lisboa

Júri:

Presidente: Professora Doutora Joana Lia Antunes Ferreira

Professora Auxiliar, Faculdade de Ciências e Tecnologia da Universidade NOVA de Lisboa

Arguente: Professora Doutora Isabel Maria Coelho de Oliveira Malaquias Investigadora Associada, Universidade de Aveiro

Vogal: Professora Doutora Márcia Gomes Vilarigues

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Setembro 2019

Beatriz Maria Barata Rodrigues

[Nome completo do autor]

Licenciada em Conservação-Restauro

[Habilitações Académicas]

From Production to Preservation: Hand-Painted Magic

Lantern Slides from the National Museum of Natural

History and Science

Dissertação para obtenção do Grau de Mestre em

Conservação e Restauro, especialização em Conservação e Restauro

Dissertação para obtenção do Grau de Mestre em

[Engenharia Informática]

Orientadora:

Professora Doutora Márcia Gomes Vilarigues

Professora Auxiliar, Faculdade de Ciências e Tecnologia da

Universidade NOVA de Lisboa

Co-orientadora:

Mestre Ângela Barros Santos

Conservadora-Restauradora, Faculdade de Ciências e

Tecnologia da Universidade NOVA de Lisboa

Júri:

Presidente: Professora Doutora Joana Lia Antunes Ferreira

Arguente: Professora Doutora Isabel Maria Coelho de Oliveira Malaquias Investigadora Associada, Universidade de Aveiro

Vogal: Professora Doutora Márcia Gomes Vilarigues

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From Production to Preservation: Hand-Painted Magic Lantern Slides from the National Museum of Natural History and Science

A Faculdade de Ciências e Tecnologia e a Universidade Nova de Lisboa têm o direito, perpétuo e sem limites geográficos, de arquivar e publicar esta dissertação através de exemplares impressos reproduzidos em papel ou de forma digital, ou por qualquer outro meio conhecido ou que venha a ser inventado, e de a divulgar através de repositórios científicos e de admitir a sua cópia e distribuição com objetivos educacionais ou de investigação, não comerciais, desde que seja dado crédito ao autor e editor.

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Acknowledgments

I want to thank first my supervisor Professor Márcia Vilarigues for embracing me in this project, motivating me to do better by myself but always being available. Thank you for letting me be myself and for all the positive criticism. It was appreciated and without you this work would not be possible.

Ângela Santos, I acknowledge the proximity you created between us, making it easy to work side by side in the laboratory. For sharing bibliography and your time while also being very understanding and encouraging in challenging moments, I thank you very much.

I am also grateful to Catarina Mateus for participating up close in this work and for sharing her knowledge with me with such great patience. Catarina Teixeira, for always helping me whenever I needed, taking the time from her busy schedule. You started to inspire me in my second year and you still do, with your generosity and never-ending experience. I would also like to thank Doctor Marta Lourenço for the shared knowledge and for making me feel welcome every time I visited the Museum. I would also like to thank Teresa Parreira and Tiago Baptista for welcoming me in ANIM and for answering all my questions with kindness and great knowledge.

I have to thank Professor Inês Coutinho for all the help with the quantifications and for being one of the best teachers I ever had, motivating me to participate in various projects and always being available to answer any question. And thank you for the glass charm! I would also like to thank Professor Joana Lia Ferreira for being so caring since the day I arrived to FCT and for being so accessible and understanding all the times. To Professor Maria João Melo, I also thank you, for pushing me to work harder and believing in my capacities.

I am so grateful for Vanessa Otero. You are bright and meticulous, and your work rhythm motivates me to strive for the best. With your amazing personality and kindness, you encouraged me like no one did, and I feel like I cannot thank you enough for all the help you gave me in the last months. I also thank Raquel Marques for helping me in the beginning of this work with the mini-review, and for giving me useful and wise insights on painting materials.

Ana Maria Martins, my sincere thank you for everything you kindly do for all of us in DCR. Bruna Primo and Carolina Peixe, thank you for making me laugh and for keeping me company during the last two years. You are amazing and I am very grateful that I met you both. A very special thanks to Rita Lourenço for understanding me like no one, for listening to me complaining for hours and for showing me what the true spirit of mutual help means, which I thought was lost. Thank you!

Andreia Pereira! You truly are the light of my life. I don’t think I have space to thank you here, but I’ll say that without you I’m sure my academic path would not have been the same. Your joyful support means the world to me and I cherish you with all my heart.

Sara Carvalho, I am so grateful for having you in my life, you inspire me more than you know, and I can only thank you for your support and the time we spent (and will keep spending) together. Daniela Cordeiro, the kindest soul I ever encountered and the most supporting person. Thank you for always believing in me and for listening to me ramble about my days. I love you both so much.

Frederico Cardoso, I want to thank you for being such a kind friend with an incredible dose of patience and for always showing interest in my work. Carolina Cunha, thank you for being such a positive friend, our friendship remains the same no matter how many years pass and that means a lot.

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Patrícia Rodrigues: thank you for being here since we were little girls and for always being a part of my life. I’m so grateful for having you close to make me laugh and to share the good (and not so good) moments of life. You always have a motivational yet realistic word to say and I love that.

A special thanks goes to Rui and Fatinha. You are one of the most amazing people I know, and I am lucky to call you family. You supported me during my all life, and I can’t thank you enough for that. To my sweet grandmother Maria, I owe you a lot and thank you for being like a second mother. My dear grandmother Ana Maria, I adore you and thank you for everything you ever done for me.

Lana and Ziggy, thank you for being the cutest company. Yogi, you’ll always have a special place in my heart.

João: thank you so much for being the best brother anyone could ever wish for. You got my back all the time and I love you so much. Thank you for all your special support through the years.

Last but definitely not least: dear parents, it’s not possible to thank you for everything but I want you to know how much I appreciate you. Thank you for making all this possible and for believing in me. I love you from the bottom of my heart and I owe it all to you.

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Abstract

The present work arises as part of the first systematic investigation on hand-painted magic lantern glass slides resorting to multi-analytical techniques combined with critical analysis of historical written sources on the painting materials and techniques used to produce them. The magic lantern was an optical instrument, used to project images from the seventeenth to the twentieth centuries, that attained great success with high impact on entertainment, science, religion and advertisement.

In the framework of this work, five hand-painted magic lantern glass slides from the National Museum of Natural History and Science (University of Lisbon) were studied. The glass support, the colourants and organic media were characterised.

The glass was analysed by Micro-Energy Dispersive X-Ray Fluorescence, and the oxide quantification unveiled that the glass belongs to the soda-lime silicate type and was possibly produced between 1870 and 1930 in England. Additionally, considering the standardized size of the slides and the similarity of the subjects represented with other English slides from the nineteenth century, it was possible to narrow the production period of this collection between 1870 and 1900.

Ultraviolet-Visible, Micro-Raman and Micro-Fourier Transform Infrared spectroscopies allowed the characterisation of the colourants. The colour palette is composed of Prussian blue, an anthraquinone red lake pigment of animal origin (such as cochineal carmine), an organic yellow whose identification was not yet possible and a carbon-based black pigment. The remaining colours – green, purple and brown – were achieved by mixing the pure pigments. Through infrared analysis, a terpenoid resin such as shellac was identified. The detection of metal carboxylates was essential to assess the state of conservation of the paints.

The identification of the main risks that might endanger the collection in study was made, as well as a risk assessment scale. Preventive conservation guidelines were proposed taking into consideration the literature on the preservation of the different materials that compose the magic lantern slides, as well as the results of surveys submitted to national and international museums that hold similar collections.

Keywords: Magic lantern glass slides; Prussian blue; Cochineal lake; Carbon-based black; Shellac resin; 19th century

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Resumo

O presente trabalho surge inserido na primeira investigação sistemática de diapositivos de lanterna mágica pintados à mão, recorrendo a uma metodologia multi-analítica combinada com a análise crítica de fontes históricas escritas acerca dos materiais e técnicas de pintura utilizados para a produção destes diapositivos. A lanterna mágica era um instrumento ótico destinado à projeção de imagens, usado desde o século XVII ao século XX. Este aparelho ótico atingiu um grande impacto em áreas como o entretenimento, a ciência, a educação, a religião e a publicidade.

No contexto deste trabalho, cinco diapositivos de lanterna mágica pintados à mão, pertencentes ao Museu Nacional de História Natural e da Ciência (Universidade de Lisboa) foram estudados. O suporte de vidro e os materiais de pintura foram caracterizados.

O vidro foi caracterizado através de Micro-Espectrometria de Fluorescência de Raio-X Dispersiva de Energia e a quantificação dos seus óxidos constituintes revelou que se trata de um vidro silicatado sodo-cálcico, possivelmente produzido entre 1870 e 1930, em Inglaterra. Adicionalmente, considerando o tamanho estandardizado dos diapositivos e a sua semelhança com outros diapositivos ingleses do século XIX, foi possível restringir o período de produção da coleção entre 1870 e 1900. A espectroscopia de Ultravioleta-Visível, Micro-Espectroscopia de Raman e de Infravermelho com Transformada de Fourier permitiram a caracterização dos colorantes. A paleta cromática é composta por azul da Prússia, uma laca vermelha de origem animal (possivelmente cochinilha), um corante amarelo orgânico cuja identificação ainda não foi possível e um pigmento preto à base de carbono. As restantes cores – verde, roxo e castanho – foram obtidas através de misturas dos pigmentos puros. A análise por espectroscopia de infravermelho permitiu identificar uma resina terpenoide, possivelmente goma-laca, bem como carboxilatos, fundamentais para a avaliação do estado de conservação das tintas.

Foi realizada a identificação dos principais riscos que poderão pôr em causa a coleção de diapositivos em estudo, assim como uma escala de identificação de riscos. Foram propostas diretrizes de conservação preventiva, considerando a literatura sobre os diferentes materiais que constituem estes diapositivos de lanterna mágica, bem como os resultados dos inquéritos submetidos a museus nacionais e internacionais detentores de coleções semelhantes.

Palavras-chave: Diapositivos de vidro de lanterna mágica; Azul da Prússia; Laca de Cochinilha; Negro à base de carbono; Resina de goma laca; Século XIX

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Index of Contents

1. Introduction ... 1

1.1. The Magic Lantern: its origin and evolution through time ... 1

1.2. Glass slides for projection with magic lantern ... 2

1.2.1. The search for movement in the slides ... 2

1.2.2. The production technique ... 3

1.2.3. Flat glass for magic lantern slides ... 3

1.3. Magic lantern slides painting: what do the historical written sources tell us? ... 4

1.3.1. Painting technique ... 5

1.3.2. Colourants mentioned in the historical written sources... 6

1.4. Case-study: MUHNAC-ULisboa’s hand-painted slides collection ... 6

2. Experimental part ... 9

2.1. Conservation assessment methodology ... 9

2.2. Material characterisation methodology ... 9

2.3. Cleaning test methodology ... 9

3. Results and Discussion ... 10

3.1. Conservation assessment ... 10

3.2. Characterisation and oxide quantification of the glass support ... 10

3.3. Characterisation of the colourants ... 12

3.3.1. Blue colour ... 12 3.3.2. Red colour ... 13 3.3.3. Yellow colour ... 14 3.3.4. Black colour ... 15 3.3.5. Green colour ... 15 3.3.6. Purple colour ... 16 3.3.7. Brown colour ... 16

3.4. Characterisation of the binder/organic media ... 17

3.5. Cleaning test ... 18

4. Preventive conservation guidelines for collections of magic lantern slides ... 19

4.1. Risk assessment and identification ... 21

4.2. Control of environmental conditions ... 22

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4.2.2. Temperature ... 23

4.2.3. Light and radiation ... 23

4.3. Guidelines for display ... 24

4.4. Guidelines for storage ... 24

5. Conclusions ... 26

6. References ... 28 7. Appendices ... I 7.1. Appendix I – Condition Reports of the Five Hand-Painted Magic Lantern Slides ... I CONDITION REPORT – HAND-PAINTED MAGIC LANTERN SLIDE UL000065 ... I CONDITION REPORT – HAND-PAINTED MAGIC LANTERN SLIDE UL000066 ... IV CONDITION REPORT – HAND-PAINTED MAGIC LANTERN SLIDE UL000067 ... VIII CONDITION REPORT – HAND-PAINTED MAGIC LANTERN SLIDE UL000068 ... XII CONDITION REPORT – HAND-PAINTED MAGIC LANTERN SLIDE UL000069 ... XVI 7.2. Appendix II – Equipment and data acquisition ... XX Stereomicroscope ... XX Energy Dispersive X-Ray Fluorescence (EDXRF) ... XX Ultraviolet-Visible Spectroscopy (UV-Vis) ... XXI Raman Spectroscopy (Raman) ... XXI Fourier Transform Infrared Spectroscopy (µ-FTIR) ... XXI 7.3. Appendix III – Average composition of the magic lantern glasses analysed via µ-EDXRF with standard deviation, in weight percent (wt.%) and parts per million (ppm) of oxides... XXII 7.4. Appendix IV – Mapping of the analysed areas ... XXIII 7.5. Appendix V – Supplementary spectra ... XXIV 7.6. Appendix VI – Conservation Survey (with answers) ... XXV

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Index of Figures

Figure 1. Hand-painted glass slide with inventory number MUL/MUHNAC UL000068 from MUHNAC-ULisboa. A) stationary image; B) movable image resulting from the operation of the slipping glass. ... 7 Figure 2. Detail (stereomicroscope photography) of A) the black background scraped off, to allow the passage of light in the areas where the brown colour was afterwards applied giving a more realistic effect to the hair (slide MUL/MUHNAC UL000065) and B) the juxtaposition of blue, red and yellow colours, creating more intricate patterns (slide MUL/MUHNAC UL000067). Note that the white particulate observed at the surface corresponds to the dirt seen under the light. ... 8 Figure 3. Details of the colours found in the hand-painted magic lantern slides studied (stereomicroscope photography). The identification of the colorants is fully discussed below, supported by the corresponding spectra. Starting from the top to the bottom, from left to right, the photos correspond to the slides MUL/MUHNAC UL000067, MUL/MUHNAC UL000068, MUL/MUHNAC UL000066, MUL/MUHNAC UL000066, MUL/MUHNAC UL000066, MUL/MUHNAC UL000066, and MUL/MUHNAC UL000069. Note that the white particulate observed at the surface corresponds to dirt seen under the light. ... 12 Figure 4. A) UV-Vis and B) Raman spectra of the blue colour, identified as Prussian blue. ... 13 Figure 5. A) UV-Vis of the red colour and B) Infrared spectra of the red colour (top) and of a 19th century W&N Crimson oil paint (bottom). These spectra strongly suggest that the colourant present is a cochineal red lake pigment; (◆) gypsum (CaSO4.2H2O). ... 14 Figure 6. Molecular structure of carminic acid [34]. ... 14 Figure 7. Raman spectrum of the black colour identified as a carbon-based black pigment. ... 15 Figure 8. A) UV-Vis spectrum of the green colour, a mixture of the organic yellow and Prussian blue; Infrared spectra of the B) purple colour, a mixture of the red and Prussian blue (); and C) brown colour, a mixture of blue, yellow and red. ... 16 Figure 9. Infrared spectra of A) the yellow and blue ( Prussian blue) colours, where the region of degradation products such as carboxylates and oxalates is highlighted; and B) the transparent resinous material found at the surface of the slide (MUL/MUHNAC UL000067) compared to a reference of shellac (Kremer). ... 17 Figure 10. Detail (stereomicroscope photography) of the dirt at the surface of the slide MUL/ MUHNAC UL000066. ... 18 Figure 11. A) before the cleaning test with the air blower and B) after the cleaning test. Only major particulate was removed. ... 19 Figure 12. Protective layered storage system for magic lantern slides collections. Adapted from [64]. ... 25 Figure 13. Hand-painted single slipping glass slide MUL/MUHNAC UL000065 from MUHNAC-ULisboa (still image) photographed with normal and transmitted light simultaneously. ... I Figure 14. Hand-painted single slipping glass slide MUL/MUHNAC UL000065 from MUHNAC-ULisboa (movable image resulting from the operation of the slipping glass) photographed with normal and transmitted light simultaneously. ... I Figure 15. Scheme of the dimensions of the magic lantern slide (in centimetres). ... II

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Figure 16. Detail (stereomicroscope photography) of A) the black background scraped off, to allow the passage of light in the areas where the brown colour was afterwards applied giving a more realistic effect to the hair and B) the detail applied in the black outline and creation of the white stripes effect, which seems to have been achieved with a thin object. ... III Figure 17. Detail showing the areas of lacuna in the black background of the slide (light green circles). ... IV Figure 18. Detail of A) the small cavity visible in the right side of the back of the wood frame and B) (stereomicroscopy photography) of the corrosion present at the metal nail. ... IV Figure 19. Hand-painted single slipping glass slide MUL/MUHNAC UL000066 from MUHNAC-ULisboa (still image) photographed with normal and transmitted light simultaneously. ... V Figure 20. Hand-painted single slipping glass slide MUL/MUHNAC UL000066 from MUHNAC-ULisboa (movable image resulting from the operation of the slipping glass) photographed with normal and transmitted light simultaneously. ... V Figure 21. Scheme of the dimensions of the magic lantern slide (in centimetres). ... V Figure 22. Detail (stereomicroscope photography) of A) the black background scraped off to reveal the green colour underneath and B) the detail applied like scratches, which seems to have been achieved with a sharp object, to add detail to wings of the bird. ... VII Figure 23. Detail showing the areas of lacuna in the black background of the slide (light green circles). ... VII Figure 24. Detail of the black and white stains on the back of the wood frame of the slide. ... VIII Figure 25. Hand-painted single slipping glass slide MUL/MUHNAC UL000067 from MUHNAC-ULisboa (still image) photographed with normal and transmitted light simultaneously. ... IX Figure 26. Hand-painted single slipping glass slide MUL/MUHNAC UL000067 from MUHNAC-ULisboa (movable image resulting from the operation of the slipping glass) photographed with normal and transmitted light simultaneously. ... IX Figure 27. Scheme of the dimensions of the magic lantern slide (in centimetres). ... IX Figure 28. Detail (stereomicroscope photography) of A) two different patterns in the blue vestment (dots and waves) and B) the juxtaposition of blue, red and yellow colours, creating a more intricate pattern. ... X Figure 29. Deposit of dirt on the slipping glass (yellow circle). ... XI Figure 30. Detail showing the areas of lacuna in the black background of the slide (light green circles) and a flaked area in the red (dark green circle). ... XI Figure 31. Photography of the back of the wood frame, showing black stains and the glued paper labels. ... XII Figure 32. Hand-painted single slipping glass slide MUL/MUHNAC UL000068 from MUHNAC-ULisboa (still image) photographed with normal and transmitted light simultaneously. ... XII Figure 33. Hand-painted single slipping glass slide MUL/MUHNAC UL000068 from MUHNAC-ULisboa (movable image resulting from the operation of the slipping glass) photographed with normal and transmitted light simultaneously. ... XIII Figure 34. Scheme of the dimensions of the magic lantern slide (in centimetres). ... XIII Figure 35. Detail (stereomicroscope photography) of A) the black background scraped off, to allow the passage of light in the areas where the brown colour was afterwards applied giving a more realistic

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effect to the hair and B) black lines drawn under the blue pattern of the sock, adding detail to the subject. ... XIV Figure 36. Detail A) (stereomicroscope photography) of dark resinous-like residues between two parts of the wood frame that seem to have been used to glue them together and B) the chipped glass, partially covered by the orange paper tape. ... XIV Figure 37. Detail showing the areas of lacuna in the black background of the slide (green circles). Detachment of the pictorial layer is marked in pink circles. ... XV Figure 38. Photography of the back of the wood frame, showing black stains, the glued paper labels as well as a piece of paper tape (top left corner) and a lacuna (top right corner). ... XVI Figure 39. Hand-painted single slipping glass slide MUL/MUHNAC UL000069 from MUHNAC-ULisboa (still image) photographed with normal and transmitted light simultaneously. ... XVI Figure 40. Hand-painted single slipping glass slide MUL/MUHNAC UL000069 from MUHNAC-ULisboa (movable image resulting from the operation of the slipping glass) photographed with normal and transmitted light simultaneously. ... XVII Figure 41. Scheme of the dimensions of the magic lantern slide (in centimetres). ... XVII Figure 42. Detail (stereomicroscope photography) of A) the black background scraped off, to allow the passage of light in the areas where the brown colour was afterwards applied giving a more realistic effect to the hair and B) the variety of techniques employed in each colour (e.g. dots in the green neck, straight lines in the brown wings) to achieve different effects and adding detail to the duck. ... XVIII Figure 43. Detail (stereomicroscopy photography) of A) the agglomerate dirt at the glass support and B) the wearing and loss of the adhesive paper tape. ... XIX Figure 44. Detail showing the areas of lacuna in the black background of the slide (light green circles). ... XIX Figure 45. A) Mapping of the analysis performed to the slide MUL/MUHNAC UL000065. UV-Vis spectroscopy analyses (): red and green paints. B) Mapping of the analysis performed to the slide MUL/MUHNAC UL000066. µ-FTIR spectroscopy analysis (●): purple paint. ... XXIII Figure 46. A) Mapping of the analysis performed to the slide MUL/MUHNAC UL000067. µ-FTIR spectroscopy analyses (●): drop of varnish and red paint. B) Mapping of the analysis performed to the slide MUL/MUHNAC UL000066. UV-Vis spectroscopy analyses (): blue and yellow paints; and µ-Raman spectroscopy analysis (◆): blue paint. ... XXIII Figure 47. Mapping of the analysis performed to the slide MUL/MUHNAC UL000069. µ-FTIR spectroscopy analyses (●): brown paint. ... XXIII Figure 48. Representative EDXRF spectrum of the glass support of the magic lantern slides (average of three measurements). ... XXIV Figure 49. UV-Vis spectra of the A) yellow and B) purple colours... XXIV Figure 50. Raman spectra of the A) green and B) brown colours. ... XXIV

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Index of Tables

Table 1. Historical written sources from the 19th century, referring to author, date, title and corresponding reference. ... 5 Table 2. Colourants mentioned in each 19th_{century written source consulted. ... 6} Table 3. Key-answers to the survey answered by the 19 participating institutions. ... 20 Table 4. Risk assessment for the MUHNAC’S magic lantern slides collection, considering each material components. Based on [62]. ... 22 Table 5. Brief summary of the preventive guidelines suggested: Establishing a compromise between the different materials of a magic lantern slide. ... 27 Table 6. Relative accuracy in percentage (%) calculated considering the CMoG B and D glass standard composition and the measured composition (wt.%). ……….…………..XX

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1. Introduction

1.1. The Magic Lantern: its origin and evolution through time

The invention of the magic lantern took place in the second half of the seventeenth century. From that moment until the last quarter of the nineteenth century, it proved to be an optical instrument able to provide knowledge on different topics as well as to provide shows that entertained a collective audience by joining the projection of images with the recitation of texts and the interpretation of melodies. This offered entertainment, mystery and adventure [1,2].

The magic lantern can actually be considered a simple device, similar to a box with an artificial light source, containing a concave mirror and a system of lens that enabled the projection of magnified images over a white surface, achieved through glasses painted with transparent colours [2].

Even though there is no date or author credited to the exact invention of the magic lantern, literature points out three crucial personalities to the description, study and development of the apparatus: it is known that it was first completely described in 1646 by Athanasius Kircher, a German Jesuit priest, in Ars Magna Lucis et Umbrae. An illustrated second edition was published in 1671 [2]. In 1659 Christian Huygens, a Dutch mathematician and astronomer developed a manuscript where it is referred a preparatory study for what seems to have been the first animated glass slide for magic lantern; despite this, Huygens only took into consideration the magic lantern in 1692, when it was already quite popular and studied, having drawn a scheme of it two years later [2,3]. The third remarkable name in the history of the origin of the magic lantern is of the Danish mathematician Thomas Walgenstein that presented it as an instrument of entertainment through all Europe since 1664 [2,3].

In 1721 William Jacob Storm Van’s Gravesande produced an improved diagram of the magic lantern with the most complete description on the technical level, only surpassed 100 years later by Phillip Carpenter, one of the most important individuals regarding the history of the magic lantern in the nineteenth century [3,4].

In a short period of time, the magic lantern established itself as an instrument worthy of attention and study, theorized, illustrated and tested by various scientists, serving multiple and versatile functions [3].

The illumination of the first magic lanterns did not allow for big public projections at the time, but this did not preclude for it from becoming a form of public entertainment. Through Europe, travelling lanternists brought the magic lantern into homes and public squares. By the end of the eighteenth century Etienne Gaspard Robertson, a Belgian showman, promoted the popularisation of the magic lantern through the Phantasmagoria shows. These performances consolidated the magic lantern as a form of entertainment, using the movement – mechanical or manual – of the lantern, making the figures seem bigger or smaller by moving the lantern closer or further away from the screen, also combining special conceived sound effects [2,4].

Precisely with the name Phantasmagoria Lantern, the English optician Philip Carpenter launched in 1821 the first important model of the magic lantern in the nineteenth century. Carpenter was also the inventor of the copper-plate sliders, slides produced by a printing process that allowed large scale production. These two inventions contributed considerably to the transformation of the magic lantern

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into an educational and recreational device, disseminated in unprecedented proportions and made economically accessible [3].

During the nineteenth century, the magic lantern was improved: the lighting was reinforced, allowing for exhibitions for a wider audience and people could now watch the shows indoors and in great halls. The performances themselves also changed, changing the subjects projected and introducing dissolving views, in which a scene could change from day to night or from one season to another. To achieve this, a biunial or triple lantern was required, enabling a scene to gradually fade away while another one was gradually introduced at the same time [2,5].

In the 1890s the first films were introduced, and they were an immediate success. For a while, the magic lantern was used at the same time: until the 1930’s magic lantern slides were used as fillers between film reels, but cinema marked the obsolescence and decline of the magic lantern [2,3]. As the importance of the magic lantern as a leisure instrument declined in the last five years of the nineteenth century – mainly due to the introduction of the cinematograph – education was still one of the few fields where the magic lantern was kept in use, serving as a visual aid for classes [1].

1.2. Glass slides for projection with magic lantern

With the projection of images and the synchronized use of sounds, the magic lantern became an audio-visual form with glass as the fundamental support to record the images. The glass can be considered as the support of the image represented, which is then projected by the magic lantern; it is called a glass slide for magic lantern.

The first known draw of a glass slide for the magic lantern is from Christian Huygens and depicts a skeleton that removes and places back his head, anticipating the search for dynamic images for the luminous projections. Starting from the subject of the death and the idea of movement, Huygens set the stage for the representation of fantastical figures, demons, skeletons and ghosts, entering in the representation of themes related to the supernatural and macabre [3].

Magic lantern slides were applied in various contexts and its use has evolved through the centuries. In the eighteenth century, scenes like fables, children's stories, mythological, allegorical and comical themes were represented in the slides [6]. During the nineteenth century, there was an increase in the subjects represented, that included sequences of popular fiction, topographical themes, historical episodes, news, educational and pedagogical subjects, as well as ludic scenes, generally combined with movement mechanisms [3,7].

1.2.1. The search for movement in the slides

Following the path of animation, other lantern rotating slides or slipping glass slides started to be produced, corresponding to the animated component of the subject. Thus, it is clear the desire and necessity to create dynamic effects in the projected images; the iconographies represented became richer and wider and it became a priority to show the projected images on the screen in action [3].

Various scholars developed different mechanisms for the movement of the glass slides, while they also evolved in terms of the subjects depicted [3].

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By the eighteenth and nineteenth centuries, the illusion of movement was achieved through different types of slides, with distinct movement mechanisms. These are fully described by Crompton et al. [8] but the more relevant ones are described below.

In the lever/rocking slide, the illusion of movement is given by the superimposing of an image of one of the glasses over the other, with one mounted glass and the other glass being movable [7]. The slipping slide consists of one of the glasses mounted into the wood frame with another glass on top with the aim of being moved horizontally/laterally in a manual way (slipping glass); the mounted glass representing the static part of the painting and on the slipping glass is painted the part of the subject intended to ‘move’. There’s still the subcategory of double slipping slide where neither of the glasses is considered stationary and both confer movement to the image [7,8]. The rackwork slide owns its movement to the action of a rack and pinion, activated by moving a crank. There are two glasses, one that spins in front of the other and they are fixed to the wood frame, normally secured with a brass ring [7,8]. The pulley slide operates with two glasses, one revolving in front of the other through a thread connected to the pulley, activated by moving a crank [8].

Additionally, please note that the slides mentioned can be subcategorized in “single” and “double”, with one or two glasses with movement, respectively [8].

1.2.2. The production technique

Three different techniques were used to manufacture the images on the glass slides for projection by magic lanterns. The first slides produced were hand-painted (discussed below) and this technique gave way to printing techniques in the first half of the nineteenth century, and in the second half, to photographic techniques, although the last two (outside the scope of this work) often included hand-colouring [6].

1.2.3. Flat glass for magic lantern slides

Flat glass was made from silica (glassformer) which melts at a very high temperature, making it necessary to add alkali oxides like potassium oxide or sodium oxide (fluxes) to lower the melting temperature. However, the chemical durability of the glass is affected by the addition of these oxides which justifies the introduction of calcium oxide or magnesium oxide (property modifiers) [9,10].

The appearance of flat glass – the type of glass used in a magic lantern slide – depends on its colour, opacity/transparency, presence of bubbles or other defects, thickness and surface finish. Other aspects of the glass result mainly from the processes used to melt the raw materials and then form the glass into thin sheets [10]. It is rare for historical flat glass to be completely colourless since sand with traces of iron was used, which gave a green or blue-green tint to the glass; a variety of materials were applied to reduce this colouring effect of the iron, namely manganese that ‘decolourizes’ the green of the iron [10]. When the glass begins to melt in the furnace it tends to contain bubbles of gas. During the refining stage, the glass is heated until it becomes fluid enough to allow these bubbles to rise to the surface, but the visual examination of most historical flat glass shows that it was practically impossible to remove the bubbles. If the melting process was flawed it could make the glass chemically inhomogeneous which visually translates into a wavy pattern due to variations in the refractive index,

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even if the glass itself is perfectly flat. Any bubbles in plate glass tend to be circular but they will be elongated in most blown or drawn glass [10].

Before the twentieth century, most of the window glass was produced using the cylinder technique or the crown technique. In either case, molten glass was collected from the furnace on the end of a hollow iron tube and the glassworker blew down the tube to produce a bubble of glass. Cylinder glass was made by elongating this bubble that was then cut open and flattened on a smooth surface. Although this technique improved with time, the resulting glass would normally have a slight roughness to one or both surfaces. Crown glass was made by opening the bloated bubble and spinning it until it formed a disk. Subsequently, crown glass had a ‘fire-polished’ surface, more often appreciated than cylinder glass. Both these techniques produced glass with slight variations in thickness, which influenced the distortion of the image transmitted. When present in large sheets, crown glass can usually be recognized by the concentric variations in thickness [10,11].

Modern flat glasses are produced with high-purity materials in carefully defined limits, but in the past, they were made with several raw materials of variable purity. Through the centuries there was a change in the chemical composition of English flat glass that seemed to have occurred abruptly since once a new technology or raw material was available, it became widely implemented by the glass industry [10]. From the end of the seventeenth century to the early nineteenth century, most window glass is of the mixed alkali type. Dungworth (2012) states this glass was made using seaweed ashes (kelp), which has a lower content of iron than HLLA (high-lime low-alkali) glass.

The most important change occurring in the glass industry during the nineteenth century was the substitution of plant ashes for synthetic soda; by 1820s the Leblanc soda technique was imported namely into England and by the late 1830s Leblanc soda displaced plant-based alkalis. This process consisted of converting common salt (NaCl) into sodium carbonate (Na2CO3). From now on, almost all window glass was of the soda-lime-silica type but it is noteworthy to mention the synthetic soda glasses can be divided into two groups: the early group (pre-1930) with high calcium and almost no magnesium, and the latter group (post-1930) with some magnesium and lower calcium content [10]. The twentieth century was marked by the search of improved mechanized production of flat glass and it was achieved in the late 1950s when Pilkington developed the float technique [10].

1.3. Magic lantern slides painting: what do the historical written sources tell us?

In order to understand the materials used and the painting techniques of the hand-painted magic lantern slides, nineteenth-century historical sources were consulted. These written sources comprise manuals with practical instructions or edited books on the magic lantern and optical projection instruments. Table 1 summarizes the historical written sources consulted.

Most of these manuals, with practical instructions or books edited about the magic lantern and optical projection apparatus, arise as publications from artists’ colourmen business, such as Winsor & Newton, Brodie & Middleton and M. J. Whipple & Co.

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Table 1. Historical written sources from the 19th century, referring to author, date, title and corresponding

reference.

1.3.1. Painting technique

Most of the historical sources consulted [12-18] indicate the colours were usually applied from tubes, which could be watercolours or oil colours, following a successive and predefined set of laborious stages that could take several days until the final painting was finished.

As mentioned by the authors, the process began with the selection of the finest thin glass plate that should have been well cleaned, and the smoother side of the glass was chosen to paint on [12,13]. The next step was the drawing of the outline1_{in the glass, that should stay undisturbed through the} subsequent painting [12]. Some authors advised to primarily cover the glass with a thin layer of varnish, diluted gelatine or a solution of sugar dissolved in water to increase the adhesion of the outline to the glass support. Others recommended using varnish to cover the painted outline to protect it [14,15]. The painting process would start by placing the glass in a retouching desk and using painting procedures and materials similar to the ones used in traditional easel painting; they began by painting the most distant subjects, followed by the foreground, and then finishing the painting [12,15]. After the necessary touch-ups, it was recommended to protect the painting by covering it with a varnish (“[…]. The varnish may be employed at the various stages, and at the finish of the work, to fix the colours.”) [10] or by placing a similar glass plate on top of the painted one [16].

The major artistic challenge of painting on magic lantern glass slides might be the great mastery necessary for the execution of the drawing and the painting since once the image is projected the most minimal details and imperfections are amplified. The other challenge arises from the transparency of the colours. The thickness of the paint layer and the amount of pigment used had a strong influence on the final result [6], and if the paint was not transparent enough it would not be possible to project the image. To achieve this transparency, the colours used to paint the magic lantern slides were necessarily less varied than the ones for traditional painting, and the authors of the written sources of the nineteenth century were aware of that [12,17,18]. Painting on magic lantern glass slides required the mastery of miniature painting on a glass substrate, and therefore, hand-painted glass slides are considered to be miniature masterpieces of cold paint on glass in their own right [6].

1_{Middleton (1876) referred the use of Indian ink for the outline to which could be added a few drops of gum water} to promote the adhesion to the glass. Hepworth (1888) advised to use black pigment for the outline and Groom (1855) indicated that the outline should be drawn with the colours that would be used to paint the distance, middle distance and foreground.

Author Date Title Reference

E. Groom 1855 The Art of Transparent Painting on Glass [12] Unknown 1856 Directions for the Graduation and Mixture of Colours, to which

are added Directions for Transparent Painting on Glass

[13] A. N. Rintoul 1867 Transparent Painting on Glass for the Magic Lantern in Water,

Oil & Varnish Colours

[17] C. Middleton 1876 Magic Lantern Dissolving View Painting [15]

W. J. Chadwick

1886 The Magic Lantern Manual [18]

T. C. Hepworth

1888 The Book of the Lantern being a Practical Guide to the working of the Optical (or Magic) Lantern

[14] W.C. Hughes 1893 The Art of Projection and Complete Magic Lantern Manual [16]

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1.3.2. Colourants mentioned in the historical written sources

Twenty-six colourants were mentioned in the nineteenth-century written sources consulted. Table 2 presents the mentioned colourants, predominately organic. Yellow colours display more variety of colourants with seven pigments, followed by brown and orange hues (six colourants). Red and black come next, with five colourants mentioned each. Two different blue colours are mentioned and just a purple colourant is mentioned. In most cases, the purple colour is described as a mixture of red and blue, but Rintoul (1888) mentions Purple Lake [14]. The green colour is not mentioned in any written source which seems to show it was obtained by mixing yellow and blue [12,13,17]. Another colour that is not mentioned by any of the authors is the white; in fact, Groom (1855) states that the white hue is represented by the absence of paint, which means white is achieved by letting the light pass through completely.

Table 2. Colourants mentioned in each 19th_{century written source consulted.}

Colourant G ro o m (18 55 ) Un kno w n (18 56 ) Rin to u l (18 67 ) Midd leto n (18 76 ) Ch adw ick (18 86 ) Hepw o rt h (18 88 ) Hu g h es (18 93 ) Asphalt ✓ Blue black ✓ Ivory black ✓ ✓ ✓ Lamp black ✓ ✓ Neutral Tint ✓ Indigo ✓ ✓ ✓ Prussian blue ✓ ✓ ✓ ✓ ✓ ✓ Brown pink ✓ Gallstone ✓ Gamboge ✓ ✓ Indian yellow ✓ ✓ Italian pink ✓ ✓ ✓ Raw Sienna ✓ ✓ Yellow lake ✓ ✓ Brown madder ✓ ✓ ✓ Burnt Sienna ✓ ✓ ✓ ✓ ✓ ✓ ✓ Burnt Umber ✓ ✓ Chinese orange ✓ Raw Umber ✓ Vandyke Brown ✓ ✓ ✓ Carmine ✓ Crimson lake ✓ ✓ ✓ ✓ Madder lake ✓ ✓ Rose madder ✓ Venetian red ✓ Purple madder ✓

1.4. Case-study: MUHNAC-ULisboa’s hand-painted slides collection

This work focuses on five hand-painted slides from the MUHNAC-ULisboa that were temporally loaned to the Department of Conservation and Restoration from FCT NOVA and Research Unit Vicarte in order to be studied. The slides were acquired in 1986 and were kept at MUHNAC’s storage. Figure

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1 illustrates one of the five magic lantern slides currently in study. The five slides are displayed in Appendix I.

A) B)

Figure 1. Hand-painted glass slide with inventory number MUL/MUHNAC UL000068 from MUHNAC-ULisboa. A)

stationary image; B) movable image resulting from the operation of the slipping glass.

The slides are denominated single slipping slides [8] since one of the glasses is stationary (mounted glass), mounted in the wood frame, and the other glass is movable to an extent (slipping glass), secured to the wood frame with metal nails. The movement of the slipping glass allows for the idea of animation intended with this type of slide.

The mounted glass has an opaque black background, and it contains most of the drawing, namely the subjects that remain still. The slipping glass on top, which appears slightly separated from the other glass by a paper tape painted in black is the one to be pulled horizontally in and out to achieve the movement action. This glass is painted with the part of the drawing that it is intended to "move" when handled; the orange tape serves the purpose of easily pulling the glass.

Under the stereomicroscope, it is noticeable that the black outline was drawn first, and the colours were applied on top. A shiny resinous-like appearance is seen over the colours and an opaque black background surrounds the subjects. This could imply the presence of a varnish or a resin-like material. It is quite interesting to notice that the white colour is achieved by the absence of paint, which allows the light to pass through entirely. Besides careful brushstrokes of the black outline, brushstrokes of a slightly darker hue of one colour are visible, giving the idea of a shadow or darker area (Figure 2B), adding detail. The intensity of the brushstroke is also varied according to the desired effect of lighter or darker details. Equally interesting is another technique observed that is similar to stained glass, namely in grisaille, which is the technique of “opening lights” in which the black background seems to be scraped off, allowing the passage of light in specific areas (Figure 2A). This demonstrates that several creative techniques and materials were employed to achieve different effects, such as the apparent application of the outline simultaneously with the black background, as well as the juxtaposition of colours to create more intricate patterns (Figure 2B).

Regarding the provenance of these objects, it is plausible to suggest they could have been produced in the nineteenth century in England, especially considering the 'standardized size' of the slides, the similarity of the subject represented [8] and the ‘wide production’ of similar depictions, with slight colour and stylistic differences that can be found in several collections attributed to this period and place of production. This suggestion is in agreement with the information present in MUHNAC's

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inventory that points the nineteenth century as the date range for the execution of these magic lantern slides.

A) B)

Figure 2. Detail (stereomicroscope photography) of A) the black background scraped off, to allow the passage of

light in the areas where the brown colour was afterwards applied giving a more realistic effect to the hair (slide MUL/MUHNAC UL000065) and B) the juxtaposition of blue, red and yellow colours, creating more intricate patterns (slide MUL/MUHNAC UL000067). Note that the white particulate observed at the surface corresponds to the dirt seen under the light.

To the extent of the author’s knowledge, there are only two studies concerning the characterisation of magic lantern slides materials [10,11]. One of them studied the presence of gum Arabic, oil, natural resins, and animal glue in different slides by means of attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) [19]. The second paper identified oil mixed with watercolour binding and resinous materials using mid-infrared fibre-optic reflectance spectroscopy [20].

The present work, an in-depth analysis of the historical glass and paints used to produce nineteenth-century hand-painted glass slides, combined with a critical analysis of the historical written sources allows, for the first time, to extend the understanding of the artistic context in which historical hand-painted magic lantern glass slides were produced.

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2. Experimental part

2.1. Conservation assessment methodology

As soon as the slides were transferred to FCT NOVA/ Vicarte facilities, photo documentation and conservation state assessment via macroscopic observation were first done, considering the different materials composing a magic lantern slide. The condition reports are displayed in Appendix II. After taking note of all the main properties of the slides, observation and photography resorting to stereomicroscope were the next step. It was possible to observe the detail of the application of the pictorial layer as well as the damages present which are better seen with ampliation. The photos taken allow for the documentation of the state of the slides before any kind of intervention or analyses.

2.2. Material characterisation methodology

One of the main goals of this work was the development of a multi-analytical experimental methodology for the material characterisation of the hand-painted glass slides, namely for the identification of the glass support, the colourants, and the organic media, using analytical techniques complementary to each other. The glass was characterised using Micro-Energy Dispersive X-Ray Fluorescence Spectrometry (µ-EDXRF) and the identification of the painting materials was performed by UV-Vis absorption spectroscopy, Micro-Raman spectroscopy (µ-Raman), Fourier Transform Infrared spectroscopy (µ-FTIR), combined with the µ-EDXRF analysis. Whenever possible, in situ analysis were given priority and micro-sampling was only employed for μ-FTIR analyses. Figures VI.1-VI.6 in Appendix VI display the micro-sampled and in-situ areas analysed.

All equipment used and data acquisition conditions are displayed in Appendix III.

2.3. Cleaning test methodology

A preliminary cleaning test was performed in a small area of the slide MUL/MUHNAC UL000069 resorting to an air blower. A CL-DF1 JJC® Professional Dust-free Air Blower was used to achieve the purpose of removing some of the potential damaging substantial dirt at the surface of the glass support. Photographic documentation via stereomicroscope was carried out before and after performing the air blowing test and is presented in section 3.5.

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3. Results and Discussion

3.1. Conservation assessment

Besides MUHNAC’s collection, Portuguese Cinematheque – Cinema Museum and National Archive of the Image in Movement magic lantern slides collection was visited. These visits allowed to expand the knowledge on different magic lantern slides, to observe the different typologies of slides and to assess the main conservation issues that affect this type of collections. Generally speaking, the major pathologies observed were breakage and/ or loss of the glass support, corrosion of the metallic parts, wearing and/or tearing of the tape (typically present in slipping slides), which could have been caused most likely due to the handling of this element, loss and/or tearing of the framing tape (typically in single slides with no wood frame), general dirt at the surface of the slide, dirt and/or stains in the wood frame and apparent presence of insects (visible through round holes in the wood frame). In what concerns the painted subjects, the main issues observed were the presence of abrasion in preferential direction (horizontal), which was most probably due to the movement of the slide during the projection, a generalized crackled pattern over the painted representation, that could possibly be attributed to the failure of the binder when exposed to aggressive projection conditions, detachment of the black opaque background paint, detachment of a specific colour – that seems to be an inherent damage to that paint formulation used – and generalized vanishing and detachment of the pictorial layer. The collection of the five slides in study, and in contradiction to the identification of degradation products via infrared analysis, is at a relatively good state of conservation, mainly displaying:

• Corrosion of the metallic nails; • Wearing and tearing of the tape; • Substantial surface dirt;

• Small-scale stains and scratches on the wood frame; • Detachment of the black opaque background paint; • Minimal detachment in the pictorial layer.

3.2. Characterisation and oxide quantification of the glass support

The elemental composition of the glasses was obtained through EDXRF analysis and three measurements were performed on each side of each of the glasses of the slide.

The spectra obtained for the analysed glasses (see the representative spectrum in Figure 49 of Appendix V) are very similar and consistent, with the same elements identified in each analysis. Therefore, it is possible to state that the five slides are composed of the same type of glass. Besides, since the elements identified in each side of the glass are in the same proportions it can also be concluded that there is no inorganic coating in any side of the glass, such as tin, which results from the float glass production method [21]. It was also possible to verify that no relevant alkali leaching, associated with the corrosion process of the glass, has occurred.

Regarding the quantitative analysis, the analysed samples belong to the soda-lime silicate glass type, containing SiO2 from 73 wt% to 75 wt%, K2O circa 0.2 wt%, Na2O + MgO from 6.5 to 9.5 wt%, and CaO from 13 to 15.5 wt%. The concentration of Al2O3 lays between 2.6 wt% and 1.3 wt%, Fe2O3 varies between 0.15-0.3 wt%, and MnO content is inferior to 0.07 wt%. Other relevant oxides are

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As2O5 (~0.05 wt%), SrO (0.02 wt%), SnO2 (0.04-0.07 wt%, ZnO (< 0.09 wt%), and TiO2 (~0.04 wt%). The complete table with the results of the quantification of all glasses in weight percentage and parts per million of oxides can be consulted in Appendix III.

Given the possible date and place of production of the five slides – England, nineteenth century – and following the investigation of one of the main researchers in English flat glass, David Dungworth, in this period three types of glass compositions will be considered for the purpose of this study.

The first type dated between c. 1700-1835 is the mixed alkali glass, known as kelp glass since it was produced using ashes of seaweed which are detectable through the high amount of strontium [11]. The average content of strontium to consider kelp ash was used is approximately 0,45±0,1% [10] which is not verified at all in the composition of the lantern glasses analysed. Phosphorus is also an important indicator since it reveals if the glass was produced using plant ashes (for phosphorus oxide percentage of c. 1,1±0,2% [10]) or if synthetic soda was used instead. Since no phosphorus was detected it is possible to suggest we are upon a glass executed with synthetic soda. In fact, Dungworth (2011) refers that since c. 1835 glasses contain little or no phosphorus in their composition, which can indicate the glass support of the magic lantern slides would have been produced afterwards that date. The glass that corresponds to the second type of composition, between c. 1835-1870, contains fewer impurities comparing to the glass made with plant ashes and doesn’t have a significant colour, possibly due to the use of purer silica sources [11]. This period, more specifically since the end of the 1830s, is marked by the production of synthetic soda (Leblanc soda) that substituted the alkalis of plants. It is important to mention that the glasses made in the first decades after the implementation of the Leblanc soda had significant arsenic concentration as this element was used as a refining agent to remove bubbles in the glass [10,11].

As of this period, almost all window glass was of the soda-lime silicate type even though it usually contained more sodium and iron than modern flat glass. In the nineteenth century, iron gave off a slightly blue-green tint to the glass, which was ‘neutralized’ with manganese and arsenic [10]. Since 1870 arsenic tends to decrease or disappear (comparatively to the last glass type, with 0,22±0,16% of As2O3); there is, however, an increase in potassium. Given the fact that the percentage of arsenic oxide in the glasses analysed is inferior to 0,2% [10] (with average values between 0,006-0,01%), it can be suggested these were produced after 1870.

The type of glass that corresponds to the third period within the date range referred, between c. 1870-1930, is very similar to what was last described. Generally, these glasses are characterised by the oxide percentage of Al2O3, SiO2, SO3, K2O, CaO, MnO, Fe2O3, As2O5 and SrO and, by comparing the content of these oxides of the literature described from 1870-1930 [10] and the results obtained for the magic lantern glasses analysed, the similarities in the composition of the glass support of the magic lantern slides studied and English flat glass from this period are clear, which allows suggesting this period as the production dating for the glass. As mentioned, the glass used for the production of the slides is of the soda-lime silicate type, produced with synthetic soda and results suggest that it is dated between 1870 and 1930, due to the low arsenic oxide content (<0.2 wt%) and potassium oxide content of circa 0.2 wt%. Further analysis to establish the precise MgO content is needed to confirm this dating. Although the production date range of the glass substrate extends into the twentieth century, it is

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possible to narrow the production range of this collection between 1870 and 1900 considering the 'standardised size' of the slides and the similarity of the subject represented with other magic lantern slides entirely produced in England [8].

3.3. Characterisation of the colourants

Figure 3 summarises the colour palette found in the five magic lantern slides under study. The identification of each colour is discussed below, divided by colour and supported by the corresponding spectra. Figures V1-V3 in Appendix V display the areas of micro-sampling and in-situ analyses presented in this work.

Figure 3. Details of the colours found in the hand-painted magic lantern slides studied (stereomicroscope

photography). The identification of the colourants is fully discussed below, supported by the corresponding spectra. Starting from the top to the bottom, from left to right, the photos correspond to the slides MUL/MUHNAC UL000067, MUL/MUHNAC UL000068, MUL/MUHNAC UL000066, MUL/MUHNAC UL000066, MUL/MUHNAC UL000066, MUL/MUHNAC UL000066, and MUL/MUHNAC UL000069. Note that the white particulate observed at the surface corresponds to dirt seen under the light.

3.3.1. Blue colour

The UV-Vis absorption spectrum of the blue colour (Figure 4A) shows an absorption band characteristic of the charge transfer transition between the Fe2+_{and Fe}3+_{ions between 600-1000 nm} [22,23].

The μ-Raman spectrum allowed the clear identification of the Prussian blue pigment (Figure 4B), displaying the characteristic stretching vibrations of the triple CN bond at 2070-2200 cm−1 [24], which are visible in the spectrum at 2152 and 2090 cm-1_{. Bands located in the spectral window between} 450-620 cm-1_{are characteristic of all Fe-C stretching vibrations [23] and manifest in the spectrum at} 523 cm-1_{. The lower spectral region (190-340 cm}-1_{) with bands at 277 and 328 cm}-1_{concern the} Fe-CN-Fe bond deformation vibrations [23,24].

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Figure 4. A) UV-Vis and B) Raman spectra of the blue colour, identified as Prussian blue.

Prussian blue is a ferric ferrocyanide (Fe4[Fe(CN)6]3) or a closely similar compound, which was one of the earliest synthetic inorganic pigments to be produced, in Berlin, 1704. It is produced by the action of an oxidising agent such as potassium bichromate and sulfuric acid on a mixture of ferrous sulphate, sodium ferrocyanide and ammonium sulphate. A deep blue pigment is precipitated from dilute solutions of those salts and it is finely divided after it is washed, filtered and washed. By controlling the precipitation and oxidation conditions as well as the addition of extenders, it is possible to obtain various hues and properties of blue [25]. Prussian blue has an extremely high tinting strength and is very transparent, ideal for glazing [26]. The origin of the colour of this pigment is an electronic transition from a low-spin Fe2+_{in a carbon coordination centre to a high-spin Fe}3+_{in a nitrogen coordination centre that} occurs when visible light is absorbed at 680 nm. This absorption – the intervalence transfer band – is responsible for the intense colour of Prussian blue [27].

3.3.2. Red colour

The UV-Vis spectra representative of the red colour in all glass slides is presented in Figure 5A. It shows an absorption band structured into two characteristic bands at 522 nm and 562 nm, indicating the presence of an anthraquinone red chromophore of animal origin (such as kermes, lac and cochineal). These bands are related to the conjugated double bonds of the anthraquinone molecule, assigned to n→π* transitions of the carbonyl groups [28,29].

Figure 5B shows the infrared spectrum of the red colour where it is possible to identify gypsum (CaSO4.2H2O) through its characteristic bands at 3405 cm-1 (OH stretching), 1621 cm-1 (H2O bending), 1125 cm-1_(SO₄2-_{asymmetric stretching) and 669 cm}-1 _(SO₄2-_{asymmetric bending) [30]. Its presence is} in agreement with the EDXRF analyses, which always detected a higher quantity of calcium, together with sulphur, in the red areas of the glass slides. The presence of a terpenoid resin such as shellac was also identified, and it is further discussed in subsection 3.3. More importantly, the infrared spectrum is very similar to a historical W&N Crimson Lake oil paint tube, which has been characterised as a cochineal red lake paint where gypsum was added during pigment manufacture [31]. Investigation into the Winsor & Newton nineteenth-century manufacturing processes for cochineal lake pigments [31,32]

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has enabled the identification of the main infrared bands attributed to the W&N Crimson lake pigment, in the form of a complex of carminic acid with aluminium and calcium, namely at 1569, 1465, 1411, 1309, 1288 and 1250 cm-1_{[32], which are also observed in Figure 5B.}

As previously noted, the use of Crimson Lake for painting on glass was advised by four out of the seven nineteenth-century sources authors. The complementary results obtained strongly suggest that the red colour is given by a cochineal red lake pigment, however, to confirm the presence of the carminic acid further analyses by Surface Enhanced Raman Spectroscopy (SERS) or High-Performance Liquid Chromatography – Diode Array Detector (HPLC-DAD) will be necessary.

Figure 5. A) UV-Vis of the red colour and B) Infrared spectra of the red colour (top) and a 19th century W&N

Crimson oil paint (bottom). These spectra strongly suggest that the colourant present is a cochineal red lake pigment; (◆) gypsum (CaSO4.2H2O).

Carmine is the generic name term applied to insect lakes – kermes and cochineal – however, due to the predominance of cochineal as an insect lake since the seventeenth century, it commonly stands for a cochineal lake [33]. Cochineal is a red dyestuff derived from various species of the scale insects Coccoidea, belonging to Dactylopiidae and

Porphyrophora. Carminic acid (C22H20O13), an anthraquinone,

is the primary dyestuff in cochineal [33]. The most common way to prepare this pigment is by precipitating a hot aqueous extract of cochineal with iron-free alum [35]. A lake pigment was typically prepared through the extraction of the dye from its animal source, in this case, and precipitating it in solution with inorganic salts (mordants) at neutral or slightly acidic pH. Mordants bind to a particular functional group in the dye molecule to form a metal-dye complex and turning the water-soluble dye into an insoluble pigment [36,37]. Carmine from cochineal has a strong red crimson hue and offers great transparency [26].

3.3.3. Yellow colour

Despite the use of complementary analytical techniques, it was not possible to identify the yellow colourant. Nevertheless, the absence of Raman signal and the similarities between the EDXRF

Figure 6. Molecular structure